A serial attached small computer system interface (SAS) system, also known as a serial attached SCSI (small computer system interface) system or a SAS domain, is a new generation serial point-to-point interface for enterprise level storage devices. The SAS system has a high expansion capability and may be connected to a large quantity of disk devices by using SAS expansion devices (also known as expansion cards). Generally, a set of devices including an SAS controller and multiple expansion cards form an SAS system. As shown in FIG. 1, an SAS controller in an existing SAS system has multiple controller ports and may be connected to multiple expansion cards. A controller port 1 in the existing SAS system is used as an example. The controller port 1 is connected to an expansion card 1, and the expansion card 1 has multiple expansion card ports and may be connected to hard disks and/or other expansion cards. For example, an expansion card port 1 is connected to an expansion card 2, and an expansion card port 2 is connected to an expansion card 3. Similar to the expansion card 1, the expansion card 2 may also be connected to multiple expansion cards and hard disks. A structure of connections between expansion cards in the entire SAS system is tree-like, and therefore, a quantity of expansion cards and hard disks in the entire SAS system can be fully expanded. Similar to the expansion card 1, the expansion card 3 may be also connected to multiple expansion cards and hard disks.
In the SAS system, if a hard disk connected to an expansion card needs to be read, for example, a hard disk connected to an expansion card 5 needs to be read, the expansion card 5 can receive a read instruction sent by the SAS controller and executes a corresponding action only after the read instruction is routed to the expansion card 2 via the expansion card 1, routed to an expansion card 4 via the expansion card 2, and then routed to the expansion card 5 via the expansion card 4. During this process, if a problem occurs in any part of a routing path from the expansion card 5 to the SAS controller, for example, the expansion card 1 becomes faulty, a communication channel between the expansion card 1 and the expansion card 2 is disconnected, or the expansion card 2 becomes faulty, the expansion card 5 cannot receive the instruction from the SAS controller, resulting in a fault in the entire system. In addition, because the SAS system has a strong expansion capability, a depth of cascading between the expansion cards may be relatively large, the probability of fault and losses due to the fault may also increase accordingly.